Local area networks (LAN's) are finding widespread use in businesses, hospitals, ships, aircraft, and the like, to serve their internal and external communications needs. In these systems, digital data and voice messages, as well as radio and television communications, are carried via a cable, or bus, which interconnects all of the sending and receiving terminals in the system.
In a broadband system, a sending terminal modulates a carrier signal with its baseband signal using, as an example, frequency division multiplexing, and transmits the modulated signal onto the bus. A receiving terminal, tuned to the frequency of the carrier signal, detects and demodulates the received signal. In this way, a single cable can carry as many simultaneous messages as there are carrier frequencies.
Each sending terminal and each receiving terminal (and those that both send and receive) are coupled to the cable via a tap. Each tap may typically include two directional couplers of the type described in U.S. Pat. No. 4,467,293, "FERRITE TYPE DIRECTIONAL COUPLER," issued Aug. 21, 1984, to T. R. Apel. The couplers may be connected to the cable to permit communications along its two directions. Such a configuration is discussed in U.S. patent application Ser. No. 685,124, entitled "LOCAL AREA NETWORK SYSTEM WITH CONSTANT TAP LEVEL," filed Dec. 24, 1984, for P. C. Basile et al.
In a typical LAN installation, the cable is laid under the flooring or in the ceiling or both. In addition, such an installation may include several senders, receivers and sender/receivers mounted in equipment racks at each location. In conventional systems, each sending and receiving station is coupled to the bus using a separate wire and cable tap, although sender/receivers usually combine their signals using combiners and splitters, as taught in the aforementioned Basile et al. reference, so that they are tied to the bus via a single tap.
In the typical system each receiver includes a preselector bandpass filter, which rejects image interference and prevents local oscillator leakage onto the bus, and a receive amplifier. Each transmitting terminal includes a low pass filter to prevent harmonics of the carrier signal frequency from entering onto the bus, since the product of harmonics, when intermixed on the bus, give inband spurious signals. Where there are several collocated receiving and transmitting terminals, it would be desirable to combine them into a grouping that would reduce the duplication of common equipment, such as preselector bandpass filters, receive amplifiers, low pass harmonic filters and cable taps. Such a grouping must, however, still permit communications among the terminals within the grouping at signal power levels compatable with intergroup communications.